The invention relates to reducing the effects of machine vibration on the accuracy of measurements made by a machine using a probe carried by the movable arm of a machine, for example a coordinate measuring machine (CMM), during a measurement process. The invention is applicable to a measurement process in which the measurements are made using a touch trigger method or a scanning method and may involve the use of analogue, digital probes or non-contact probes.
One method of 3D surface scanning involves moving a scanning probe attached to the quill of a CMM over the surface of an article to be measured, and noting the coordinates of the position of a workpiece-contacting stylus of the scanning probe at a number of points on the surface of the article. The motors of the machine are provided with speed demand signals from the controller, and it is usual to have servo-loops within the controller which obtain position and velocity feedback from each axis of the machine to control the motor speeds. Velocity feedback is derived by means of a tachogenerator on the motor, and positional feedback is obtained by means of a linear encoder system on each axis of the CMM.
The current method however does not take into account the dynamics of the CMM. It assumes that all the axes of the CMM are rigid, and therefore that the feedback from the tachogenerators and linear encoders are sufficient for accurate servo control of the position of the scanning probe.
However, CMMs are not infinitely rigid structures. Most have flexible structural members onto which the probe is mounted, and these are subject to vibration, produced by the drive motors and external disturbances such as air bearing instability and mechanical linkages. These vibrations, either by themselves or by exciting the resonant frequencies of the machine structures, can give rise to the scanning probe losing contact with the surface being scanned, or inaccuracies in the measurements being taken, resulting in reduced measuring accuracy and increased scanning time.
In the scanning process described above, scanning time can be reduced if the coordinate measuring machine can be run at higher speed, and if less time is spent on surface recoveries, i.e. repeating movements where the scanning probe left the surface.
In a touch trigger measurement process during which the machine is stopped each time the probe contacts a workpiece, measurement speed can be increased if the machine could be allowed to move around more quickly and to stop and start more quickly with less vibrations.
Various methods have been used in the past to try and eliminate or at least reduce the effects of such vibrations.
For example, vibrations can arise from the frequency response of the various structures of the machine to frequencies induced in the drive motors by the demand signal, particularly if the demand voltage is in the form of a step function.
One commonly practised method is to reduce the overall feedback gain of the position and velocity servo loops.
Reducing servo loop gain will reduce the frequency bandwidth of the control system i.e. the range of frequencies over which control can be maintained, and will result in the loss of servo performance, e.g. large position overshoot, poor position tolerance. Another commonly used method of combatting structural resonance induced by the drive motors is to introduce a notch filter as part of the velocity loop, or in the input to the velocity servo controller. This notch filter is tuned to the dominant resonant frequency of the machine. This method is limited however in that it removes movements only at one particular frequency and may also reduce the servo bandwidth for that axis. However, it is possible for a coordinate measuring machine axis to have more than one resonant frequency.
It is also known from U.S. Pat. No. 5,594,668, to establish parameters characterising elastic bending behaviour of the CMM for several probe positions within the measuring range. This is done by determining components of the parameters which are dependent upon the position of the probe on the machine slides and on acceleration forces acting on the slides, and storing details of these components as correction values for subsequent use in correcting measurements made on workpieces.
These stored values are obtained by measuring the acceleration of the machine""s slides during a measuring process, or by determining the acceleration from the positional data generated by the measuring systems of the machine, and differentiating the positional data twice according to time. The disadvantage of this method however, is that it is not dynamic insofar as the correction values are generated during a calibration process and stored in a multi-dimensional correction table. In order to minimise the number of points required to be calibrated to construct this table, the ability to interpolate between points stored in the table is an essential requirement of the system.
It is also known from UK Patent No. 2,045,437, to provide accelerometers in a probe which is mounted on the quill of a CMM, to determine from the measured accelerations of the quill what the resultant deflection of the quill is, and to correct the probe reading from such deflections.
Once again however, this process is not dynamic insofar as it requires a calibration process to establish in a memory store, a table of deflections for different accelerations at different positions within the measuring volume of the machine, and the actual measurements taken during a measuring process are then corrected by reference to correction values taken from the memory store.
An object of the present invention is to provide a dynamic method of reducing acceleration-produced errors in machine measurements.
Another object of the present invention is to provide apparatus which performs the above method without reducing the range of frequencies over which good servo control over the machine movements can be maintained.
In accordance with the present invention, there is provided a method of reducing measurement errors made by a machine using a probe comprising the steps of:
deriving acceleration signals indicative of accelerations of moving parts of the machine,
deriving from the acceleration signals, velocity signals indicative of the change in velocity of the moving part caused by the accelerations, and
using the velocity signals in a velocity feedback control loop to provide correction signals to the machine to reduce the effects of acceleration-induced deflections of machine parts on the measurements made by the machine.
It may be possible on some axes of the machine to derive the acceleration signals by measuring the displacement of the respective moving part of the machine and differentiating the displacement twice with respect to time.
Preferably however, the accelerations of the moving parts are measured directly using accelerometers placed in association with the respective parts.
The accelerometers may be positioned on the readheads which measure the movements of the machine axes, but the best results are obtained when the accelerometers are positioned on or close to the probe, e.g. in the probe body, on the head on which the probe is mounted, on the machine quill to which the head is mounted, or even on the probe stylus.